diff --git a/problem52.py b/problem52.py
new file mode 100644
index 00000000..67ed330d
--- /dev/null
+++ b/problem52.py
@@ -0,0 +1,23 @@
+class BSTIterator:
+    def __init__(self, root):
+        self.st = []
+        self.dfs(root)
+
+    def dfs(self, root):
+        while root:
+            self.st.append(root)
+            root = root.left
+
+    def next(self):
+        temp = self.st.pop()
+        self.dfs(temp.right)
+        return temp.val
+
+    def hasNext(self):
+        return len(self.st) > 0
+
+# Time Complexity:
+# amortized O(1)
+
+# Space Complexity:
+# O(h)
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diff --git a/problem53.py b/problem53.py
new file mode 100644
index 00000000..eedef58d
--- /dev/null
+++ b/problem53.py
@@ -0,0 +1,50 @@
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+from typing import Optional
+
+
+class Solution:
+    def reorderList(self, head: Optional[ListNode]) -> None:
+        """
+        Do not return anything, modify head in-place instead.
+        """
+        if not head or not head.next:
+            return
+
+        slow = head
+        fast = head
+
+        while fast and fast.next:
+            slow = slow.next
+            fast = fast.next.next
+
+        #now slow will point to the middle node
+        prev = None
+        curr = slow.next
+
+        while curr:
+            temp = curr.next
+            curr.next = prev
+            prev = curr
+            curr = temp
+
+        slow.next = None
+
+        l1 = head
+        l2 = prev
+
+
+        while l2:#l2 will be slower or same size
+            temp1 = l1.next
+
+            l1.next = l2
+            l2 = l2.next  
+            l1.next.next = temp1
+
+            l1 = temp1
+
+# Time Complexity: O(N)
+# Space Complexity: O(1)
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diff --git a/problem54.py b/problem54.py
new file mode 100644
index 00000000..e69de29b
diff --git a/problem55.py b/problem55.py
new file mode 100644
index 00000000..fabc78d9
--- /dev/null
+++ b/problem55.py
@@ -0,0 +1,46 @@
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, x):
+#         self.val = x
+#         self.next = None
+
+class Solution:
+    def getIntersectionNode(self, headA: ListNode, headB: ListNode) -> Optional[ListNode]:
+
+        if not headA or not headB:
+            return
+
+        curr = headA
+        lengthA = 0
+
+        while curr:
+            lengthA += 1
+            curr = curr.next
+
+        lengthB = 0
+        curr = headB
+
+        while curr:
+            lengthB += 1
+            curr = curr.next
+
+        slow = headA
+        fast = headB
+
+        while lengthA > lengthB:
+            slow = slow.next
+            lengthA -= 1
+
+        while lengthB > lengthA:
+            fast = fast.next
+            lengthB -= 1
+
+
+        while slow != fast:
+            slow = slow.next
+            fast = fast.next
+
+        return slow        
+    
+# Time Complexity : O(m+n)
+# Space Complexity: O(1)
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